Protecting Group Chemistry Research Articles

ChemInform Abstract: Weapons in Disguise—Activating Mechanisms and Protecting Group Chemistry in Nature

AbstractReview: [78 refs.]

Formate dehydrogenase - a biocatalyst with novel applications in organic chemistry

In the field of industrial biocatalysis, formate dehydrogenase (FDH) is well established, in particular for its broad application in cofactor regeneration. Further applications have been limited by the enzyme's narrow range of substrates. These restrictions have been overcome now by the finding, that the enzyme is capable of selectively cleaving formic acid esters to the respective alcohol. Five homologous alkyl formates and phenyl formate as an aromatic ester were converted quantitatively by FDH from Candida boidinii in a batch reaction within 3 to 5 h. The substrates were turned irreversibly into carbon dioxide and the respective alcohol through hydride abstraction from the formyl group with full conversion. The mechanism shows parallels to hydrolysis reactions of the A(AC)1-type. K(M)-values and reactions rates of the tested formic acid esters display a tendency to higher conversion rates with increasing chain length. FDH emerged to be a superior deformylation catalyst compared to hydrolases as well as classical catalysts, as was shown by the selective deformylation of 1-acetoxy-4-formoxy butane (92%) and 1,3-bis(3-formoxypropyl)tetramethyldisiloxane. In particular its capability to distinguish between formic acid esters and non-formic acid esters renders the method particularly suitable for protective group chemistry. Furthermore the completeness of deformylation allows for converting substrates highly incompatible with aqueous media like siloxanes within a few hours.

Facile, modular transformations of RAFT block copolymers via sequential isocyanate and thiol-ene reactions

We describe a robust strategy utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization and sequential transformations involving carbamate formation and thiol-ene click addition to synthesize well-defined functional block copolymers. The hydroxy-functional block copolymer scaffold, poly[(N,N-dimethylacrylamide)-b-(N-(2-hydroxyethyl)acrylamide)] (PDMAn-b-PHEAm) was first prepared via RAFT, requiring no protecting group chemistry. The hydroxyl groups of the HEA block were then reacted with 2-(acryloyloxy)ethylisocyanate (AOI) or allylisocyanate (AI) resulting in acrylate- and allyl-functionalized copolymer precursors, respectively. The efficiencies of both Michael and free radical-mediated thiol-ene addition reactions were investigated using model thiol compounds having alkyl, aryl, hydroxyl, carboxylic acid, amine and amino acid functionalities. The steps of RAFT polymerization, isocyanate-hydroxyl coupling and thiol-ene addition can be accomplished under mild conditions, thus offering a facile, modular route to the synthesis of functional copolymers from a single polymeric precursor.

Weapons in disguise--activating mechanisms and protecting group chemistry in nature.

Bioactive natural products often possess uniquely functionalized structures with unusual modes of action; however, the natural product itself is not always the active species. We discuss molecules that draw on protecting group chemistry or else require activation to unmask reactive centers, illustrating that nature is not only a source of complex structures but also a guide for elegant chemical transformations which provides ingenious chemical solutions for drug delivery.

ChemInform Abstract: Preactivation‐Based, One‐Pot Combinatorial Synthesis of Heparin‐Like Hexasaccharides for the Analysis of Heparin—Protein Interactions.

Heparin (HP) and heparan sulfate (HS) play important roles in many biological events. Increasing evidence has shown that the biological functions of HP and HS can be critically dependent upon their precise structures, including the position of the iduronic acids and sulfation patterns. However, unraveling the HP code has been extremely challenging due to the enormous structural variations. To overcome this hurdle, we investigated the possibility of assembling a library of HP/HS oligosaccharides using a preactivation-based, one-pot glycosylation method. A major challenge in HP/HS oligosaccharide synthesis is stereoselectivity in the formation of the cis-1,4-linkages between glucosamine and the uronic acid. Through screening, suitable protective groups were identified on the matching glycosyl donor and acceptor, leading to stereospecific formation of both the cis-1,4- and trans-1,4-linkages present in HP. The protective group chemistry designed was also very flexible. From two advanced thioglycosyl disaccharide intermediates, all of the required disaccharide modules for library preparation could be generated in a divergent manner, which greatly simplified building-block preparation. Furthermore, the reactivity-independent nature of the preactivation-based, one-pot approach enabled us to mix the building blocks. This allowed rapid assembly of twelve HP/HS hexasaccharides with systematically varied and precisely controlled backbone structures in a combinatorial fashion. The speed and the high yields achieved in glycoassembly without the need to use a large excess of building blocks highlighted the advantages of our approach, which can be of general use to facilitate the study of HP/HS biology. As a proof of principle, this panel of hexasaccharides was used to probe the effect of backbone sequence on binding with the fibroblast growth factor-2 (FGF-2). A trisaccharide sequence of 2-O-sulfated iduronic acid flanked by N-sulfated glucosamines was identified to be the minimum binding motif and N-sulfation was found to be critical. This provides useful information for further development of more potent compounds towards FGF-2 binding, which can have potential applications in wound healing and anticancer therapy.

Approaches for the Synthesis of Functionalized Cryptophycins

The first syntheses of bioactive cryptophycins functionalized at unit D were accomplished in a one-pot Staudinger reduction/cyclization step. An azido precursor for the lower part of the backbone was introduced to minimize protective group chemistry and enable a very convenient synthesis of cryptophycin-52 and unit D cryptophycin analogues containing an ester or a free carboxylic acid for bioconjugations. Both new cryptophycin derivatives show high biological activity in cytotoxicity assays.

Preactivation-based, one-pot combinatorial synthesis of heparin-like hexasaccharides for the analysis of heparin-protein interactions.

Heparin (HP) and heparan sulfate (HS) play important roles in many biological events. Increasing evidence has shown that the biological functions of HP and HS can be critically dependent upon their precise structures, including the position of the iduronic acids and sulfation patterns. However, unraveling the HP code has been extremely challenging due to the enormous structural variations. To overcome this hurdle, we investigated the possibility of assembling a library of HP/HS oligosaccharides using a preactivation-based, one-pot glycosylation method. A major challenge in HP/HS oligosaccharide synthesis is stereoselectivity in the formation of the cis-1,4-linkages between glucosamine and the uronic acid. Through screening, suitable protective groups were identified on the matching glycosyl donor and acceptor, leading to stereospecific formation of both the cis-1,4- and trans-1,4-linkages present in HP. The protective group chemistry designed was also very flexible. From two advanced thioglycosyl disaccharide intermediates, all of the required disaccharide modules for library preparation could be generated in a divergent manner, which greatly simplified building-block preparation. Furthermore, the reactivity-independent nature of the preactivation-based, one-pot approach enabled us to mix the building blocks. This allowed rapid assembly of twelve HP/HS hexasaccharides with systematically varied and precisely controlled backbone structures in a combinatorial fashion. The speed and the high yields achieved in glycoassembly without the need to use a large excess of building blocks highlighted the advantages of our approach, which can be of general use to facilitate the study of HP/HS biology. As a proof of principle, this panel of hexasaccharides was used to probe the effect of backbone sequence on binding with the fibroblast growth factor-2 (FGF-2). A trisaccharide sequence of 2-O-sulfated iduronic acid flanked by N-sulfated glucosamines was identified to be the minimum binding motif and N-sulfation was found to be critical. This provides useful information for further development of more potent compounds towards FGF-2 binding, which can have potential applications in wound healing and anticancer therapy.

Chemical Approaches for Studying Histone Modifications

Histones form the protein core around which genomic DNA is wrapped in eukaryotic chromatin. Numerous genetic studies have established that the structure and transcriptional state of chromatin are closely related to histone post-translational modifications. Further elucidation of the precise mechanistic roles for individual histone modifications requires the ability to isolate and study homogeneously modified histones. However, the highly heterogeneous nature of histone modifications in vivo poses a significant challenge for such studies. Chemical tools that have enabled biochemical and biophysical studies of site-specifically modified histones are the focus of this minireview.

Synthesis of Functional Polydepsipeptides via Direct Ring‐Opening Polymerization and Post‐Polymerization Modification

AbstractThis contribution explores different strategies for the synthesis of side chain functional polydepsipeptides. First, the ring‐opening polymerization of side chain functional morpholine‐2,5‐diones is revisited and the optimized reaction conditions used for the polymerization of (Z)‐L‐Lys, (Boc)‐L‐Lys and L‐allylglycine based morpholine‐2,5‐diones. As a first approach towards side chain functional polydepsipeptides, the deprotection of poly(Glc‐alt‐(Z)‐L‐Lys) and poly(Glc‐alt‐(Boc)‐L‐Lys) is evaluated. Although under appropriate conditions, the side chain protecting groups can be quantitatively removed, the reaction conditions used here were found to lead to backbone degradation. As an alternative approach, the thiol‐ene post‐polymerization modification of poly(Glc‐alt‐allylglycine) is explored. Free radical addition of various ω‐functional thiols was found to proceed without backbone degradation and in several cases with quantitative allyl group conversion. The post‐polymerization modification strategy is attractive as it obviates the need for protecting group chemistry and facilitates the synthesis of diverse libraries of side chain functional polydepsipeptides.magnified image

Atom transfer radical polymerization synthesis and association properties of amphiphilic pullulan copolymers grafted with poly(methyl methacrylate)

AbstractAmphiphilic copolymers of pullulan grafted with poly(methyl methacrylate) (PMMA) were synthesized by atom transfer radical polymerization under homogeneous mild conditions without using protecting group chemistry. The hydroxyl groups of pullulan were reacted with 2‐bromoisobutyryl bromide to prepare pullulan macroinitiators with various degrees of substitution. Kinetic study showed that the polymerization was first order. The copolymers were characterized using 1H NMR spectroscopy and gel permeation chromatography. The molecular weights of the grafted chains were controlled and polydispersities were low. Association properties in aqueous solution were studied using 1H NMR spectroscopy, dynamic light scattering and transmission electron microscopy. Spherical nanoparticles with size and size distribution significantly affected by the number and length of the grafted chains were formed. Graft copolymers with a degree of substitution of 5.3% and length of PMMA grafted chains from 5 to 35 repeating units formed well‐defined quite monodisperse spherical nanoparticles with hydrodynamic diameters in the range 20–40 nm. This means that nanoparticle size can be tuned by changing the length of the grafted chains for this degree of substitution. Less control of aggregate size was obtained for a degree of substitution of 1.0%. Copyright © 2010 Society of Chemical Industry

Total synthesis of vinigrol.

The longstanding challenge posed by the complex diterpene vinigrol has been answered for the first time. The notorious difficulty in synthesizing vinigrol stems from its unprecedented decahydro-1,5-butanonaphthalene ring system, eight contiguous stereocenters, and highly congested functionality. This Communication delineates a stereocontrolled 23-step route to vinigrol that is scalable (>5 g prepared of a late-stage intermediate), minimally reliant on protecting group chemistry, and facilitated by a number of unique and chemoselective transformations.

A Post‐Modification Strategy for the Synthesis of Uniform, Hydrophilic/Hydrophobic Patterned α‐Hydroxy Acid Oligomers

AbstractHydrophilic/hydrophobic patterning is a well‐established design strategy to guide secondary structure formation of both natural as well as non‐natural oligomers and polymers. This contribution explores the feasibility of a new approach for the synthesis of uniform, sequence‐defined, hydrophilic/hydrophobic patterned oligo(α‐hydroxy acid)s. The proposed strategy is based on post‐modification of a reactive oligoester scaffold composed of an alternating sequence of hydrophobic [(2S)‐2‐hydroxy‐4‐methylpentanoic acid] and masked hydrophilic [(2S)‐2‐hydroxypent‐4‐enoic acid] α‐hydroxy acids. The use of (2S)‐2‐hydroxypent‐4‐enoic acid instead of a complex side‐chain‐protected hydrophilic building block obviates the need for additional protective group chemistry during chain extension. In a subsequent post‐modification step, the allyl side chains can be quantitatively modified via free‐radical addition of different ω‐functional thiols to afford hydrophilic/hydrophobic patterned oligoesters. The proposed synthetic strategy provides an interesting alternative to rapidly generate libraries of foldamers with identical chain length and monomer sequence but different side‐chain functionalities.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

ATRP synthesis and association properties of temperature responsive dextran copolymers grafted with poly( N-isopropylacrylamide)

Temperature responsive copolymers of dextran grafted with poly( N-isopropylacrylamide) (Dex- g-PNIPAAM) were prepared by atom transfer radical polymerization (ATRP) in homogeneous mild conditions without using protecting group chemistry. Dextran macroinitiator was synthesized by reaction of dextran with 2-chloropropionyl chloride at room temperature in DMF containing 2% LiCl. ATRP was carried out in DMF:water 50:50 (v/v) mixtures at room temperature with CuBr/Tris(2-dimethylaminoethyl)amine (Me 6TREN) as catalyst. Several grafted copolymers with well defined number and length of low polydispersity grafted chains were prepared. Temperature induced association properties in aqueous solution were studied as a function of temperature and polymer concentration by dynamic light scattering, fluorescence spectroscopy and atomic force microscopy (AFM). LCST, ranging from 35 to 41 °C, was significantly affected by number and length of grafted chains. The fine tuning of LCST around body temperature is an important characteristic not obtainable by conventional radical grafting of PNIPAAM. Well defined spherical nanoparticles were formed above the LCST of PNIPAAM. Hydrodynamic diameter was in the range 73–98 nm.

ChemInform Abstract: New Strategies for Protecting Group Chemistry: Synthesis, Reactivity, and Indirect Oxidative Cleavage of para‐Siletanylbenzyl Ethers.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

End‐functionalized polystyrene by ATRP: A facile approach to primary amino and carboxylic acid terminal groups

AbstractMonoamino‐terminated and monocarboxylic acid‐terminated polystyrenes containing active halogenated end groups were prepared by atom transfer radical polymerization (ATRP) using the so‐called initiator method and protective group chemistry. α‐Chloropropionates were synthesized and utilized as initiators containing the tert‐butoxycarbonyl (t‐BOC)‐protected amino and the tert‐butyl (t‐Bu)‐protected carboxylic acid function, respectively. Optimum polymerization conditions were attained using CuCl/N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) as catalyst and 10 vol % n‐butanol as homogenizing agent at 110 °C. However, targeting larger quantities an alternative route was established employing 50 vol % N,N‐dimethylformamide (DMF). Subsequent hydrolysis of the ω‐tert‐butoxycarbonyl polystyrenes afforded well‐defined polymers with quantitative deprotection of the functional groups. Comparatively, thermolytic cleavage of the protective sites was studied. 1H NMR verified the quantitative removal of the t‐BOC‐protecting groups. Furthermore, the resulting α‐amino‐ω‐chloro polystyrenes were reacted with Sanger reagent to confirm the existence of the thereby converted primary amino groups. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3845–3859, 2009

New Strategies for Protecting Group Chemistry: Synthesis, Reactivity, and Indirect Oxidative Cleavage of para-Siletanylbenzyl Ethers

Reported herein is a new entry in the growing arsenal of arylmethyl ether protecting groups. The para-siletanylbenzyl (PSB) ether is electronically similar to the benzyl ether. Cleavage of the PSB ether is accomplished under mild conditions--involving alkaline hydrogen peroxide--that are unique among cleavage protocols for arylmethyl ethers. Furthermore, the PSB group affords the user new flexibility in the implementation of protecting group strategies that revolve around multiple arylmethyl ether protecting groups. In addition to hydrogen peroxide-based cleavage protocols, conversion of a PSB ether into a para-methoxybenzyl (PMB) ether and assembly of a PSB ether from a pre-existing para-bromobenzyl (PBB) ether are described. Finally, a new reagent for installing PSB ethers under neutral "mix and heat" conditions is reported.

Simple Synthesis of 3-Acetamido-β-resorcylic Acids as Potential FabF and FabH Inhibitors without Using Protecting Groups

A simple two-step strategy for the synthesis of 3-acet­amido-β-resorcylic acids as potential platensimycin analogues was developed. It avoids the use of protecting group chemistry and starts from 2-aminoresorcinol, which is first N-acylated and then subjected to a modified Kolbe-Schmitt carboxylation to yield the desired 3-acetamido-β-resorcylic acids.

MALDI‐TOF MS Investigations of Polyamides Synthesized in Autoclave and Microwave Assisted Processes

AbstractDue to their excellent physical and chemical properties polyamides (PA) are used in many industrial processes. The most common products are the aliphatic polyamides synthesized by a conventional melt‐polycondensation method of AH‐salt in an autoclave. High temperatures, high pressure and long reaction times are necessary. On the other hand the polyamide synthesis can conducted as solution polycondensation in a microwave oven. Moreover the higher heating efficiency of microwave irradiation reduces the reaction time to about 20 minutes. The main focus of this study lies on the end group analysis by MALDI‐TOF MS (matrix assisted laser desorption/ionization time of flight mass spectrometry) of aliphatic polyamides produced by autoclave – as well as microwave assisted synthesis by AABB‐type condensation. The results show differences in the end group formation depending on the chosen reaction conditions. In case of following modification or protecting group chemistry, it is of course important to know the kind of end groups of the polyamide.

Graft copolymers prepared by atom transfer radical polymerization (ATRP) from cellulose

A cellulose-based macro-initiator, cellulose 2-bromoisobutyrylate, for atom transfer radical polymerization (ATRP) was successfully synthesized by direct homogeneous acylation of cellulose in a room temperature ionic liquid, 1-allyl-3-methylimidazolium chloride, without using any catalysts and protecting group chemistry. ATRP of methyl methacrylate and styrene from the macro-initiator was then carried out. The synthesized cellulose graft copolymers were characterized by FTIR, 1H NMR and 13C NMR spectroscopies. The grafted PMMA and PS chains were obtained by the hydrolysis of the cellulose backbone and analyzed by GPC. The results obtained from these analytical techniques confirm that the graft polymerization occurred from the cellulose backbone and the obtained copolymers had grafted polymer chains with well-controlled molecular weight and polydispersity. Through static and dynamic laser light scattering and TEM measurements, it was found that the cellulose graft copolymer in solution could aggregate and self-assembly into sphere-like polymeric structure.

Total Synthesis of (±)-Psychotrimine

new process for the union of anilines with tryptamine derivatives has been developed, furnishing C-3 quaternized pyrroloindoline architectures. This chemoselective coupling of a tryptamine with 1.2 equiv of 2-iodoaniline proceeds efficiently on a gram-scale using only the simple reagents N-iodosuccinimide and triethylamine. Using this new reaction, the complex natural product psychotrimine has been fashioned with a rare level of efficiency and practicality. From a readily available bromotryptamine, only four steps (41-45% overall isolated yield) are necessary to procure gram quantities of the natural product. Functional group manipulations, protecting group chemistry, and unnecessary redox fluctuations have been minimized.